Oral vaccine for swine dysentery and method of use

ABSTRACT

An oral preparation for increasing the resistance of swine to swine dysentery infection comprises enteric-coated orally-administrable pellets containing concentrated killed cells of a virulent isolate of Treponema hyodysenteriae. In the preferred method of use, the preparation is orally administered to swine in a plurality of doses providing at least 3 mg. of the cells per animal per dose.

An anaerobic spirochete, Treponema hyodysenteriae, has beencharacterized as the primary etiological agent in swine dysentery.Harris, D. L.; Glock, R. D.; Christensen, C. R.; and Kinyon, J. M.: Vet.Med./Small Animal Clin. 67:61 (1972); Taylor, D. J.; and Alexander T. J.L.: Brit. Vet. J. 127:108 (1971). But relatively little is known aboutthe immunology of swine dysentery although resistance to reinfection canbe demonstrated in convalescent pigs. In 1976, Glock et al reported thatparenteral vaccination of pigs with killed cells of a virulent isolateof T. hyodysenteriae provided a significant degree of protection againstsubsequent intragastric challenge with live virulent T. hyodysenteriae.Glock, R. D., Schwartz, K. J., and Harris, D. L., Proceedings,International Pig Veterinary Society Congress, June 1976, Ames, Iowa.The vaccine was given in six intravenous injections at 6-day intervals.This was the first reported success in immunizing swine against swinedysentery infection. For field use, an oral vaccine would be moreconvenient to use. However, previous attempts to develop an oral vaccinehave produced discouraging results.

Hudson et al found that oral dosing of an attenuated strain of T.hyodysenteriae provided no protection against subsequent challenge.Hudson, M. J., Alexander, T. J. L., Lysons, R. J., Wellstead, P. D.,Brit. Vet. J. (1974) 130:37. Subsequently, Hudson et al attempted toimmunize pigs with live attenuated T. hyodysenteriae using a combinationof oral dosing and parenteral inoculation. Hudson, M. J., Alexander, T.J. L., Lysons, R. J., Prescott, J. F., Res. Vet. Science (1976) 21:366.Oral doses were administered on three consecutive days, and after aninterval of several days, intraperitoneal vaccinations wereadministered, which were followed after several more days byintramuscular vaccinations. The overall results of these tests weresummarized as follows: "Although vaccination appeared to enhanceimmunity to swine dysentery, half of the vaccinated pigs developed thedisease. This level of protection would be unlikely to be of practicalvalue in the field."

SUMMARY OF INVENTION

The present invention is based in part on the discovery that theresistance of swine to dysentery infection can be increased by orallyadministering enteric-coated pellets containing concentrated killedcells of a virulent isolate of Treponema hyodysenteriae. The entericcoating is selected so that it will be resistant to dissolving in theswine stomach while dissolving in the swine intestines to release thekilled cells for immunizing action. Preferably, the enteric coating issubstantially insoluble in water at a pH below 5.0, while becomingslowly soluble in water at a pH of 5.8 to 6.2. Oral preparationsprepared in accordance with the present invention are particularlyuseful for administration to young pigs which are subject to swinedysentery infection. For this purpose, the oral vaccine is prepared inthe form of enteric-coated granules, which are mixed with afinely-divided feed material, and fed to pigs on the basis of a regimenwhich provides massive introduction of the killed cells into the colonarea. Preferably, the immunizing feed is administered to the pigs atleast onece every 24 hours for a period of at least five days with eachfeeding providing at least 3 milligrams of the cells (dry basis) peranimal.

Other features and advantages of the invention will be described in thefollowing specification.

DETAILED DESCRIPTION

The present invention can be practiced with any virulent isolate of T.hyodysenteriae. Attenuated or non-virulent isolates or strains are notdesirable. A virulent isolate or strain is one which is capable ofproducing a typical swine dysentery infection. One suitable isolate hasheretofore been identified in the literature as B204. See Kinyon, J. M.,and Harris, D. L.: Vet. Rec. (1974): 95:219. The same publication alsorefers to an isolate identified as B234, which can also be used inpracticing the present invention. However, type strain B78 (ATCC No.27164) is not suitable, being nonvirulent. Isolates B204 and B234 havebeen deposited with the American Type Culture Collection; B204 beingidentified as ATCC No. 31212 and B234 as ATCC No. 31287. It should beunderstood that these isolates are representative of class of virulentisolates or strains which can be employed.

The T. hyodysenteriae cells for preparation of the oral vaccine can becultured using trypticase soy broth (TSB) with 10% (v/v) fetal calfserum (FCS). For example, the inoculated broth can be incubated at37°-38° C. under an anaerobic atmosphere, such as 50:50 H₂ :CO₂ or CO₂alone. The gaseous atmosphere should be deoxygenated. For furtherdetails, see Kinyon, J. M., and Harris D. L.: Vet. Rec. (1974): 95:219.

After the fermentation has been completed, the cells can be recoveredand concentrated by centrifugation or ultrafiltration to obtain a cellslurry for further processing. The cells are killed by a suitableprocedure either in the fermenter or after recovery. Standard killingagents may be used such as formalin or merthiolate. For example, akilling-concentration of formalin, such as 0.2% formalin (v/v), can beadded to the fermenter or cell slurry. The slurry of killed cells isused to prepare the enteric-coated pellets.

To act as a filler or bulk stabilizer for desiccation and pelletpreparation, standard filler substances may be added to the cell slurrysuch as sucrose, dextrose, lactose, etc. In general, the amount offiller-stabilizer to be added may range from about 10 to 50 parts byweight of filler per 100 parts of cells (dry basis). Prior to theaddition of the filler, the cell concentrate preferably contains inexcess of 3.0 milligrams of cells (dry basis) per millileter of slurry.For example, the cell concentrates may contain from about 4 to 7milligrams of cells (dry basis) per milliliter of cell slurry. Theparticular concentration is not critical, since most of the residualwater of the slurry is removed by a suitable drying procedure inpreparing the pellets.

For example, a 1:10 dilution of sterile 50% non-fat powdered milk plus50% H₂ O solution to act as filler-stabilizer was added to theconcentrated slurry (4.0 mg/ml dry weight antigen). This material wasdried by a biological drying procedure such as freeze-drying.

The cell slurry containing the added filler may be dried by a biologicaldrying procedure such as freeze-drying. Preferably, the drying iscarried out at a relatively low temperature, such as not over 40° C. Thedried material is then pulverized to a finely-divded condition forpreparing tablets or granules. If desired, a binder material such asstarch may be added at this point, although the filler may provideadequate binding. Tablets may then be prepared by tableting in astandard tablet press, either manual or automatic types.

Alternatively, the dried cells may be formed into granules by a knowngranulation procedure. For example, the cell concentration in the formof a liquid slurry produced by ultrafiltration is mixed with sucrose andcellulose, and kneaded to a doughy consistency. The cell dough is thenextruded in the form of noodles or ribbons, which are broken up andformed into granules by a granulation apparatus. The granule size is notcritical, but desirably is of a small size, such as below 20 mesh (U.S.Standard Screen). The granules are dried in an oven at a relatively lowtemperature, such as 37°-40° C. until most of the moisture has beenremoved. The final moisture content is not critical, and desirably mayrange from about 1 to 3% water by weight.

Orally-administrable pellets, which may comprise tablets or granules asdescribed above, are provided with an enteric coating for the purpose ofthe present invention. The material used to provide the enteric coatingshould be selected so that it is resistant to dissolving in the swinestomach, while dissolving in the swine intestines. The differences in pHbetween the stomach and intestines can be used to control the dissolvingof the enteric coating. While the pH conditions of the stomach andintestines of swine vary with diet and time of day, in general, the pHof a swine stomach and of the gastric fluid is below 5.0, such as 4.0 to4.5. The pH of the small gut may range from about 5.5 in the upper smallgut to about 5.9 to 6.1 in the middle and lower small gut. Similarly,the pH of the large bowel and colon may range from about 5.8 to 6.1.Preferably, therefore, the enteric coatings used in the presentinvention are substantially insoluble in water at a pH below 5.0, whilebeing slowly soluble in water at a pH of 5.8 to 6.2. With thismechanism, the coatings are not exposed to the pH conditions of themouth long enough to be dissolved and the coatings can remainsufficiently intact in the stomach to protect most of the vaccine fromdestruction by the gastric juice. During transit through the smallbowel, the coatings on the granules are gradually dissolved, and arecompletely dissolved or disintegrated at least by the time the vaccinereaches the colon. The killed cells are therefore released in the swineintestines for immunizing action in the swine colon.

When the swine are being fed continuously, such as with feeder pigs, thepH of the food mass in the stomach may vary from about 4.0 near thestomach wall to 5.7 to 6.1 at the center of the food mass. The averageresidence time of the food in the stomach may range from 1 to 2 hours.Preferably, therefore the coatings on the vaccine granules shouldrequire at least one hour to dissolve in water at pH 6.0 and 37° C. (thestandard temperature).

Any enteric coatings which meet the foregoing pH conditions can beutilized in practicing the present invention. One enteric coatingmaterial which can be used is cellulose acetate phthalate. As is wellknown in the art, cellulose acetate phthalate may be plasticized withdiethyl or dibutyl phthalate so that the coating is more resistant tocracking. For application, the enteric coating material may be dissolvedin a suitable volatile organic solvent, and the enteric coat may bebuilt up in a series of applications to assure that the coating will becomplete and relatively uniform. One known procedure of this kind isreferred to as the "Open-Pan Ladle Coating Process." For example, 30 to40 parts by weight of cellulose acetate phthalate together with 8 to 10parts of diethyl phthalate may be dissolved in 250 to 300 parts byweight of acetone to form a coating solution. Suitable enteric coatingresins may also be used, such as acrylic resins prepared for use asenteric coatings. One such product is sold under the trademark "EudragitL" by Rohm Pharma Gmbh, Darmstadt, West Germany. The release pH ofEudragit L can be increased where desired by mixing it with Eudragit S.The manufacturer describes Eudragit L as soluble in intestinal juicefrom pH 6.0 and Eudragit S as soluble from ph 7.0. These enteric coatingmaterials are supplied in granular form containing 10% moisture;specified as Eudragit L90 and Eduragit S90 to indicate the 90% activecontent. Eudragit L or mixtures of L and S are soluble in alcohols andacetone which may be used for applying the coating. The coatingmaterials may be plasticized with various plasticizers, such aspolyethylene glycol, dibutylphthalate, triacetin, or castor oil.Satisfactory coating results, however, can be obtained using theEudragit material without plasticizer addition.

The Wurster Coating Process can also be used to apply the entericcoatings. This process is described in U.S. Pat. Nos. 3,241,520 and3,253,944. It is carried out as a commercially available service byCoating Place, Inc., Verone, Wis.

Where the swine are being fed continuously, a preferred coating containsfrom 5 to 20 parts by weight of Eudragit S in admixture with from 95 to80 parts of Eudragit L. For example, 15 parts S with 85 parts Lincreases the release pH so that at least 75% and up to 90% of thecoated granules pass through the stomach with the protective coatingssubstantially undissolved. There is complete release in the intestinesbecause of the longer residence time (8 to 24 hours).

The enteric-coated oral vaccine of the present invention is preferablyin the form of granules which can be readily mixed with swine feedmaterial for administration to the animals. For example, such granulesmay range from about -20 mesh to +100 mesh (U.S. Standard Screen). Thegranules are mixed with a finely-divided feed material such as a groundfeed used for baby pigs after weaning. Any swine or pig feed materialcan be used, such as a basal ration containing ground corn, rolled oats,soybean meal, minerals, and vitamins. The coated granules may also bemixed with vitamin-mineral fortified premixes which are later combinedwith the other feed ingredients by the pig raisers. Such pre-mix mayalso contain high protein feed materials such as soybean meal.

While the enteric-coated vaccine pellets of the present invention andthe oral method of immunization may be applied to adult swine, such asbreeding sows, an important use is with growing pigs. For example, themethod may be applied to feeder pigs as soon as they are receiving solidfeed, either shortly before or following weaning. For example, thevaccine administration may be started at the age of about 3 to 8 weeks.The method may also be applied to older pigs during their growth periodprior to marketing. The pigs raised under field conditions are highlysubject to swine dysentery infection with consequent economic loss dueto lowering of the rate of weight gain and the feed efficiency. Byincreasing the resistance of the pigs to swine dysentery infection,optimum rates of weight gain may be maintained.

In practicing the present invention, it is desirable to administer aplurality of doses of the enteric-coated oral vaccine, such as from 3 to15 doses. Preferably, such dose should provide at least 3 milligrams ofthe killed cells of T. hyodysenteriae (dry basis) per animal; the dosesbeing administered daily (once every 24 hours), such as by admixture ofthe enteric-coated granules with a feed material. This dosing may becontinued for a period of at least 5 days, such as from 5 to 15 days. Ina preferred embodiment, the oral doses contain at least 4 mg. of thekilled cells (dry basis) per dose, such as doses in the range of 4 to 6mg. Since 1 mg. of the cells (dry basis) contains about 2.5×10⁹ cells, adose of 5 mg. will provide approximately 1.2×10¹⁰ cells. Therefore, inaccordance with the present invention, massive quantities of the killedcells are delivered to the swine colon for immunizing action therein.

In applying tablet and granule coatings, a dye may be included as acolorant for the coating. This permits the coating to be more readilyinspected for thickness and uniformity, and makes it easier to detectimperfections in the coatings. In practicing the present invention, itis desirable to use a dye in the enteric coatings of the presentinvention, although it is not essential with respect to the desiredimmunizing action. Suitable dyes include Lake Blue No. 2 and crystallineviolet dye.

This invention is further illustrated by the following examples.

EXAMPLE I

Organism: The microorganism, Treponema hyodysenteriae, Strain 204 (ATCCNo. 31287) was grown in Trypticase Soy Broth containing 5-10% sterileFetal Calf Serum (GIBCO Laboratories, Grand Island, N.Y.) in test tubes,500 cc Erlenmeyer flask, 4-liter glass jugs and in a 25-liter pilot NewBrunswick fermentor. The growth culture was maintained until log phasegrowth ceased. The culture was inactivated by addition of formalin to0.2% concentration. 50:50 H₂ :Co₂ or no gas was used to maintain thegaseous atmosphere in the fermentor during the growth period. After thefermentation had been completed, the cells were concentrated in liquidmass by ultrafiltration.

The cell culture was forced by positive air pressure through a 100,000molecular weight size membrane. The cell slurry was concentrated to afinal concentration of 4.0 milligrams dry weight measurement per ml ofculture. Ten percent (10%) by volume sterile milk stabilizer, (50%Non-fat dry milk powder in 50% H₂ O) was added to the antigen T.hyodysenteriae slurry. E.g. 750 ml culture at 4 milligrams antigen/ml or3.0 gram of antigen added to 75 ml milk stabilizer containing 37.5 gramsdry powdered milk and 37.5 ml H₂ O. The antigen milk stabilizer mixturewas freeze-dried in a commercial freeze drier for 18 hours. Thetemperature was never allowed above 37° C. during the drying of themixture. 66.20 grams of dried material was recovered from the dryingprocess.

Capsule Preparation: Ten percent (10%) by weight dry soluble starch wasadded to the dried material giving the following components:

3.0 grams active antigen T. hyodysenteriae (killed cells-dry basis)

0.3 grams powdered milk stabilizer

0.33 grams soluble starch binder

0.5 Gram mixture (3 mg dry basis active T. hyodysenteriae antigen) ofthe above mixture was placed in a hand-operated pellet press. A 3/8"punch and die set was used to give a tablet of approximately 3/8"thickness by 1/2" in diameter.

Each tablet was coated with an enteric coating which upon contact in achemical environment pH 6 or greater, will degrade and release theantigenic material into the colonic lumen. The coating is not soluble atthe stomach pH.

Enteric Delivery System:

1. Enteric Coating Solution--(36.0 grams of cellulose acetate phthalate,9.0 grams of diethyl phthalate, and 0.2 grams of crystalline violet dye)was diluted into 255 grams of reagent grade acetone. (For a descriptionof a similar procedure, see C. J. Malm, J. Amer. Phar. Assoc, ScienceEdition, Vol. 40, p. 520, 1951.)

2. Enteric Coat Application--Each tablet was air blasted to remove dustand was submerged under the surface of the coating solution at leasttwenty times, with complete air drying of tablets between coatings.

3. Wax Coating--The enteric coating was further protected from H₂ Ouptake by application of a white wax ("Be Square" 195, PetroliteCorporation, Bareco Division, Tulsa, Okla). This was liquified in the GItract, and did not inhibit the antigen release in the colon. Ten (10)grams of "Be Square" 195 was beads were dissolved in 10 ml of acetone.Each enteric coated tablet was submerged at least twice in the liquidwax solution. The wax coat was allowed to dry prior to tablet bulkingand storage. Stearic acid or other fatty acid or fatty acid triglyceridecan also be used as an auxilliary coating to further protect the vaccinein the stomach. The fatty acids and triglycerides will be digested offthe granules in the small gut.

Parenteral Vaccines:

The test also included administration of parenteral vaccine prepared asfollows: The concentrated liquid slurry described above (4.0 mg. T.hyodysenteriae cells dry basis per ml) was diluted from 4.0 mg/ml to 1.0mg/ml with a 20% solution of sterile aluminum hydroxide adjuvant. Theresulting parenteral vaccine contained about 2.5×10⁹ killed cells permilliliter.

Materials and Methods

Experimental Animals--Sixteen (16) pigs from a herd with no history ofswine dysentery were placed in swine testing facilities at approximatelyfive weeks of age and fed a 16% protein grown ration containing nodrugs.

Preparation of Vaccines--An orally delivered enteric release vaccine anda parenteral vaccine containing Treponema hyodysenteriae Isolate B204(ATCC No. 31287) with aluminum hydroxide as adjuvant was prepared.

Preparation of Inoculum--Cultures of Treponema hyodysenteriae (IsolateB204) were grown approximately 24 hours in aerobically preparedtrypticase soy broth containing 10% fetal calf serum under deoxygenatedH₂ :CO₂ at 37° C. Seventy-five (75) ml of whole culture containingapproximately 5×10⁸ organisms per ml was administered to each pig viastomach tube following a 48 hour starvation period. The isolate of T.hyodysenteriae had not been passaged more than eight (8) times in vitro.

Experimental Design--The sixteen (16) pigs were randomly assigned toindividual pens. The pigs were immunized as follows:

    ______________________________________                                        No. of    Parenteral Vaccine  Oral                                            Group Pigs    Subcutaneous*                                                                             Intraperitoneal*                                                                        Vaccine*                                  ______________________________________                                        I     4       -           +         +                                         II    4       +           -         +                                         III   8       -           -         -                                         ______________________________________                                         *+ = administered; - = not administered.                                 

On day 0, the pigs in Group I were injected with 5ml/pig of theparenteral vaccine. Pigs in Group II were injected with a similaramount/pig by subcutaneous route. On day 14, a 5ml/pig boostervaccination was given by respective routes. On day 19, each pig inGroups I and II were given one (1) oral tablet per day through the 29thday. On day 30, all pigs were weighed. Feed was withheld on days 30 and31. On day 32, rectal swabs were collected and all pigs were challengedwith T. hyodysenteriae as described above. Clinical evaluation ofresponse to challenge was recorded for each pig on a twice daily basisfor 40 days post inoculation. On day 40 post-inoculation, each pig wasweighed.

Evaluation of Response to Challenge--Each pig was observed twice dailyand 3 clinical parameters were scored on a scale of 1 to 3. The resultsare shown in Table A.

                  TABLE A                                                         ______________________________________                                        Clinical Response  Group                                                      ______________________________________                                                           I        II      III                                       Diarrhea:          N=4.sup.a                                                                              N=4     N=8                                        Day of Onset.sup.b /Post Inocul.                                                                16       5.2     13.6                                       Days Duration      4       8.2     11.8                                       No. Affected       4       4        6                                        Dysentery:                                                                     Day of Onset/Post Inocul.                                                                       25.7     24.2    17.6                                       Days Duration      3.2      2.5     5                                         No. Affected       2        2       5                                        ______________________________________                                        General Condition:                                                                          1 = Normal                                                                    2 = Gaunt, mildly inactive                                                    3 = Emaciated, moribund                                         Feces Consistency:                                                                          1 = Normal, firm                                                              2 = Soft, not formed                                                          3 = Liquid                                                      Feces Composition:                                                                          1 = Normal                                                                    2 = Increased mucus                                                           3 = Large amount of blood present                               ______________________________________                                         .sup.a N equals number of pigs per group                                      .sup.b Study terminated at 40 days. Calculations are based on a value of      40 assigned to each pig which remained normal.                           

Observation of T. hyodysenteriae Like Organisms--Rectal swabs werecollected. A drop of each sample was reviewed under a dark fieldmicroscope observation. There was significant increase in the numbers ofT. hyodysenteriae like organisms in clinically affected pigs which wasconsidered evidence of an ongoing swine dysenteriae infection.

Weight Gain--The average weight gain during the period from day ofchallenge inoculation to the day of study termination was comparedbetween vaccinated and non-vaccinated groups. The results are shown inTable B.

                  TABLE B                                                         ______________________________________                                                  Group                                                                         I         II       III                                              ______________________________________                                        Days                                                                          Post Test Initiation                                                                      IP-Oral     SC-Oral  Controls                                      0          26.0        27.6     31.1                                         40          103.0       110.2    94.0                                         Total Average Gain                                                                        77.0        82.6     62.9                                         per pig                                                                       ______________________________________                                    

EXAMPLE II

Enteric coated granules can be prepared by concentrating fermentor grownTreponema hyodysenteriae culture to about 64 mg dry weight/ml. Usingsuch a concentrated antigen slurry it is combined with the otheringredients in the following proportions:

1500 cc antigen slurry

11.5 kilo sucrose

3.5 kilo microcrystalline cellulose

0.2% dry weight Lake blue No. 2 dye

H₂ o added as needed for obtaining proper texture

Once this mixture is partially mixed the moistened mass of material isrun through a commercial extruder at least three times to provide auniform mix of antigen to carrier. The cylindrical pieces are thenshaped into uniform beads in a manumerizer. The bead preparation isdried at least 1-8 hours leaving 1-3% moisture content. An entericcoating is then applied to the beads by either the Wurster or anopen-pan, ladle type coating process, as previously identified. Apreferred coating is a mixture of 85 parts by weight of Eudragit L 90with 15 parts Eudragit S 90, using acetone or ethanol as the carriersolvent for applying the coating. (Eudragit L 90 and S 90 are sold byRohm Pharma Gmbh, Darmstadt, West Germany.)

The enterically coated beads prepared as described will containapproximately 10% containing immunizing antigen (dry basis) can beadministered in the following manner:

A. remove all feed from swine (viz. weaned baby pigs) to be immunized 24hours prior to treatment.

B. mix the enteric coated granules with 10-20% of the daily requiredfeed in a 1:1 ratio.

C. each pig whould receive an average of 6-10 mg immunizing antigen/day,which determines the average amount of feed to be given to a group ofpigs.

D. feed untreated feed free choice during part of the day, but withholdfeed overnite.

E. repeat this feeding and withholding procedure for 5-10 days.

EXAMPLE III

Antigenic vitamin-mineral premixes can be prepared by mixing theimmunizing granules prepared as described in Example II with standardpig feed fortification premixes. For example, the granules containing10% of the T. hyodysenteriae cells (dry basis) are mixed with thefollowing premix in the amount 100 grams granules per 7.5 pounds of avitamin-mineral premix. The following premix is illustrative:

    ______________________________________                                        Vitamin A USP UNITS    600,000 per lb. of premix                              Vitamin D3                                                                              USP UNITS    600,000 "                                              Vitamin E I Units      200     "                                              D-Calcium mg.          800     "                                              Pantothenic                                                                   Niacin    mg.          1,600   "                                              Niacin    mg           4,000   "                                              Chlorine  mg.          20,000  "                                              Chloride                                                                      Vitamin B 12                                                                            mg.          2       "                                              B.H.T.    mg.          22,680  "                                              Manganese mg.          10,886  "                                              Zinc      mg.          10,000  "                                              Iron      mg.          3,628   "                                              Copper    mg.          362     "                                              Iodine    mg.          225     "                                              Cobalt    mg.          36      "                                              L-Lysine  mg.          2,500   "                                              Antigenic gr.          13.3    "                                              Graules (10%)                                                                 ______________________________________                                    

The above antigenic premix is then incorporated in pig feed in theamount of 7.5 pounds per ton. The following basal ration formula isillustrative:

    ______________________________________                                        1307     Corn (ground #2)                                                     200      Rolled Oats                                                          435      Soybean Meal (44% solvent)                                           25       Di-Calcium Phosphate                                                 15       Feeding Lime                                                         10       Iodized Salt                                                         + 7      1/2 Antigenic Vitamin-Mineral Premix                                 2000     lbs.                                                                 ______________________________________                                    

The foregoing example and data illustrate the oral administration ofenteric-coated tablets for increasing the resistance of swine to swinedysentery infection. The companion parenteral administration can beomitted and the oral administration used alone. However, the combinationprocedure is believed to enhance the effectiveness of the oraladministration, as described in the co-pending application Ser. No.935,062 of Delbert L. Harris and Robert A. Goodnow, filed on even dateherewith, and entitled "Method of Increasing the Effectiveness of OralVaccination for Swine Dysentery."

I claim:
 1. An oral preparation for increasing the resistance of swineto swine dysentery infection, comprising enteric-coatedorally-administrable pellets containing concentrated killed cells of avirulent isolate of Treponema hyodysenteriae, said enteric coating beingresistant to dissolving in the swine stomach while dissolving in theswine intestines to release said cells for immunizing action.
 2. Thepreparation of claim 1 in which said killed cells of Treponemahyodysenteriae are prepared from isolate B204 (ATCC No. 31287).
 3. Anoral preparation for increasing the resistance of swine to swinedysentery infection, comprising enteric-coated orally-administrablepellets containing concentrated killed cells of a virulent isolate ofTreponema hyodysenteriae, said enteric coating being substantiallyinsoluble in water at a pH below 5.0 while being slowly soluble at a pHof 5.8 to 6.2, the coatings on said pellets requiring at least one hourto dissolve in water at 37° C. and pH 6.0.
 4. The method of increasingthe resistance of swine to swine dysentery infection, characterized byorally administering to swine while free of active swine dysenteryinfection a plurality of doses of enteric-coated pellets containingconcentrated killed cells of a virulent isolate of Treponemahyodysenteriae, said enteric coating being resistant to dissolving inthe swine stomach while dissolving in the swine intestines, and saiddoses providing at least 3 milligrams of said cells (dry basis) peranimal per dose.
 5. The method of claim 4 in which said killed cells ofTreponema hyodysenteriae are prepared from isolate B204 (ATCC No.31287).
 6. The method of increasing the resistance of pigs to swinedysentery infection, said pigs being free of active swine dysenteryinfection but subject thereto, comprising preparing enteric-coatedgranules containing concentrated killed cells of a virulent isolate toTreponema hyodysenteriae, said enteric coating being resistant todissolving in the swine stomach while dissolving in the swineintestines, mixing said granules with a finely-divided feed material forpigs, and feeding said mixture to the pigs at least once every 24 hoursfor a period of at least 5 days, each of said feedings providing atleast 3 milligrams of said cells (dry basis) per animal.
 7. The methodof claim 6 in which said killed cells of Treponema hyodysenteriae areprepared from isolate B204 (ATCC No. 31287).
 8. The method of claim 6 inwhich said enteric coating is substantially insoluble in water at a pHbelow 5.0 while being slowly soluble in water at a pH of 5.8 to 6.2, thecoatings on said granules requiring at least one hour to dissolve inwater at 37° C. and pH 6.0.
 9. The method of claim 8 in which saidkilled cells of Treponema hyodysenteriae are prepared from isolate B204(ATCC No. 31287).
 10. An antigenic premix containing vitamins andminerals for administration to growing pigs characterized by alsocontaining enteric-coated granules containing concentrated killed cellsof a virulent isolate of Treponema hyodysenteriae, said enteric coatingbeing resistant to dissolving in the swine stomach while dissolving inthe swine intestines to release said cells for immunizing action. 11.The antigenic premix of claim 10 in which said killed cells of Treponemahyodysenteriae are prepared from isolate B204 (ATCC No. 31287).
 12. Theantigenic premix of claim 10 or claim 11 in which said enteric coatingis substantially insoluble in water at a pH below 5.0 while being slowlysoluble at a pH of 5.8 to 6.2, the coatings on said granules requiringat least one hour to dissolve in water at 37° C. and pH 6.0.